He’s also taken £170 from our sons account and still not paid it back. Son isn’t aware yet he would be devastated. It’s all his birthday money saved up.
Please make an effort to stop him from using your son's account as a slush fund. A friend of mine discovered that her ex had stolen thousands of dollars from their son's savings account (the son had inherited the money from another relative). She finally took him to court and won.
Also, I would make sure that he hasn't stolen any of your children's identities to score additional credit cards/loans/etc.